1. Field of the Invention
The present invention relates to a method for manufacturing a monolithic piezoelectric part and the monolithic piezoelectric part itself, and more particularly, relates to a method for manufacturing a monolithic piezoelectric part such as a monolithic piezoelectric actuator, a monolithic piezoelectric audio emitter or a monolithic piezoelectric sensor, wherein the layers are reduced in thickness and increased in number, and a high piezoelectric d constant (distortion constant) is required, and the monolithic piezoelectric part manufactured by the above method.
2. Description of the Related Art
In recent years, piezoelectric parts such as piezoelectric actuators, piezoelectric audio emitters, piezoelectric sensors, piezoelectric transformers, and so forth, which use the piezoelectric properties of ceramic materials, have come into widespread use for mobile communication equipment, audio-visual equipment, office automation equipment, and so forth.
Further, recent development of monolithic piezoelectric parts is being actively undertaken from the perspective of reducing the size of electronic parts and improving the piezoelectric properties thereof, and particularly, attempts are being made to reduce the size of electronic parts and improve the piezoelectric properties thereof by reducing the thickness of the layers of the ceramic sheets which make up the monolithic piezoelectric parts and increasing the number of the ceramic sheets.
However, internal electrode materials such as Ag disperse within the ceramic body with arrangements wherein the layers of monolithic piezoelectric parts are reduced in thickness or increased in number, leading to deterioration of the piezoelectric properties thereof and poorer reliability.
To deal with this problem, a technique wherein a ceramic other than a piezoelectric article and a material for internal electrodes are sintered together in a low-oxygen atmosphere has been proposed as a method for suppressing dispersion of the internal electrode material such as Ag within the ceramic body (Japanese Patent No. 2,676,620, Japanese Examined Patent Application Publication No. 6-20014, Japanese Unexamined Patent Application Publication No. 2-122598).
The aforementioned documents disclose a sintering process in a low-oxygen atmosphere (e.g., with oxygen concentration of 50,000 ppm or lower) which reduces the activation of Ag serving as the internal electrode material, and thus suppresses dispersion of Ag to the ceramic body at the time of sintering.
Also, a technique has been proposed wherein the amount of Ag dispersed is controlled by the concentration of oxygen within the furnace at the time of sintering. (Japanese Unexamined Patent Application Publication No. 11-163433 and Japanese Unexamined Patent Application Publication No. 11-274595).
These documents disclose a hard-type piezoelectric ceramic material with a small piezoelectric d constant and a high mechanical quality coefficient Qm as a material for a piezoelectric transformer or the like, wherein good transformer properties are obtained by making the A site component volume of the perovskite compound oxide expressed with a general formula of ABO3 greater than that of a stoichiometric composition, using an internal electrode material wherein the ratio by weight of Ag and Pd is within the range of 60/40 to 80/20 for Ag/Pd, and sintering within an atmosphere wherein the oxygen concentration is 1% or more.
However, in the event that the technique disclosed in Japanese Patent No. 2,676,620, Japanese Examined Patent Application Publication No. 6-20014 and Japanese Unexamined Patent Application Publication No. 2-122598 is applied to a Pb perovskite piezoelectric ceramic material, the piezoelectric d constant is conspicuously deteriorated since sintering within a low-oxygen atmosphere promotes generation of oxygen pores. Particularly, deterioration of the piezoelectric d constant is so remarkable when the oxygen concentration is reduced to less than 1% by volume (hereafter, the unit “% by volume” will be represented by “vol %”) or when a soft piezoelectric ceramic material with a high piezoelectric d constant is used, that its application to monolithic piezoelectric parts such as monolithic piezoelectric actuators, monolithic piezoelectric audio emitters, monolithic piezoelectric sensors, or the like, wherein a high piezoelectric d constant is required is difficult.
In Japanese Unexamined Patent Application Publication No. 11-163433 and Japanese Unexamined Patent Application Publication No. 11-274595, an internal electrode material wherein the ratio by weight of Ag and Pd is within the range of 60/40 to 80/20 for Ag/Pd is used, but in the event that the amount of Ag, which is less expensive than Pd, is increased to 80% by weight (hereafter, “% by weight” will be represented by “wt %”) in order to reduce the cost of the electrode material, the amount of Ag dispersion also increases, so that formation of oxygen pores is promoted and the piezoelectric d constant and the insulation resistance are probably deteriorated.
While Japanese Unexamined Patent Application Publication No. 11-163433 and Japanese Unexamined Patent Application Publication No. 11-274595 disclose a monolithic piezoelectric transformer obtained by layering ceramic sheets of 80 to 100 μm in thickness, further reduction in thickness of the ceramic sheets also increases the amount of Ag dispersion, which leads to further deterioration of the piezoelectric properties and the insulation resistance. These publications also disclose use of a hard-type piezoelectric ceramic material with a high mechanical quality coefficient Qm since the primary object is a piezoelectric transformer, but a piezoelectric actuator, a piezoelectric audio emitter, a piezoelectric sensor, and so forth, require use of a soft piezoelectric ceramic material with a high piezoelectric d constant. However, there has been the problem that with such soft piezoelectric ceramic materials, when the amount of Ag contained in the internal electrode material is increased, the thickness of the layers of ceramic sheets is reduced or the number of the layers thereof is increased, formation of oxygen pores is further promoted by dispersion of Ag within the piezoelectric ceramic particles and sintering within a low-oxygen atmosphere, to result in a marked reduction in the piezoelectric d constant.